asked 1 month ago
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Questions have many tags and tags have many questions. I can see two options:
Is there a reason to prefer one when I need "questions for a tag" and "tags for a question" to both be fast?
Do both, and accept the duplication.
The embedded array makes rendering a question cheap — one document, no join. The join collection makes "all questions for tag X" cheap and gives you somewhere to hang a compound unique index:
The cost is that writes must update both, so wrap them in a transaction. For a read-heavy site that trade is almost always worth it.
One caveat on the embedded array: keep a denormalised questions counter on the tag itself. Otherwise the tags page has to run a countDocuments per tag, which gets slow fast.
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